Piezoelectric materials and magnetostrictive materials (collectively referred to below as xe2x80x9cpiezoelectric materialsxe2x80x9d) deform when an electric field or magnetic field is applied. Thus piezoelectric materials, when used as an actuator, are capable of controlling the relative position of two surfaces.
Piezoelectricity is the general term to describe the property exhibited by certain crystals of becoming electrically polarized when stress is applied to them. Quartz is a good example of a piezoelectric crystal. If stress is applied to such a crystal, it will develop an electric moment proportional to the applied stress.
This is the direct piezoelectric effect. Conversely, if it is placed in an electric field, a piezoelectric crystal changes its shape slightly. This is the inverse piezoelectric effect.
One of the most used piezoelectric materials is the aforementioned quartz. Piezoelectricity is also exhibited by ferroelectric crystals, e.g. tourmaline and Rochelle salt. These already have a spontaneous polarization, and the piezoelectric effect shows up in them as a change in this polarization. Other piezoelectric materials include certain ceramic materials and certain polymer materials. Since they are capable of controlling the relative position of two surfaces, piezoelectric materials have been used in the past as valve actuators and positional controls for microscopes. Piezoelectric materials, especially those of the ceramic type, are capable of generating a large amount of force. However, they are only capable of generating a small displacement when a large voltage is applied. In the case of piezoelectric ceramics, this displacement can be a maximum of 0.1% of the length of the material. Thus, piezoelectric materials have been used as valve actuators and positional controls for applications requiring small displacements.
Two methods of generating more displacement per unit of applied voltage include bimorph assemblies and stack assemblies. Bimorph assemblies have two piezoelectric ceramic materials bonded together and constrained by a rim at their edges, such that when a voltage is applied, one of the piezoelectric materials expands. The resulting stress causes the materials to form a dome. The displacement at the center of the dome is larger than the shrinkage or-expansion of the individual materials. However, constraining the rim of the bimorph assembly decreases the amount of available displacement. Moreover, the force generated by a bimorph assembly is significantly lower than the force that is generated by the shrinkage or expansion of the individual materials.
Stack assemblies contain multiple layers of piezoelectric materials interlaced with electrodes that are connected together. A voltage across the electrodes causes the stack to expand or contract. The displacements of the stack are equal to the sum of the displacements of the individual materials. Thus, to achieve reasonable displacement distances, a very high voltage or many layers are required. However, conventional stack actuators lose positional control due to the thermal expansion of the piezoelectric material and the material(s) on which the stack is mounted.
Due to the high strength, or stiffness, of piezoelectric material, it is capable of opening and closing against high forces, such as the force generated by a high pressure acting on a large surface area. Thus, the high strength of the piezoelectric material allows for the use of a large valve opening, which reduces the displacement or actuation necessary to open or close the valve.
With a conventional piezoelectrically actuated relay, the relay is xe2x80x9cclosedxe2x80x9d by moving a mechanical part so that two electrode components come into electrical contact. The relay is xe2x80x9copenedxe2x80x9d by moving the mechanical part so that the electrode components are no longer in electrical contact. The electrical switching point corresponds to the contact between the electrode components of the solid electrodes.
Liquid metal micro switches have been developed that use liquid metal as the switching element and the expansion of a gas when heated to actuate the switching function. The liquid metal has some advantages over other micromachined technologies, such as the ability to switch relatively high power approximately 100 mW) using metal-to-metal contacts without microwelding, the ability to carry this much power without overheating the switch mechanism and adversely affecting it, and the ability to latch the switching function. However, the use of a heated gas to actuate the switch has several disadvantages. It requires a relatively large amount of power to change the state of the switch, the heat generated by switching must be rejected effectively if the switch duty cycle is high, and the actuation speed is relatively slow, i.e., the maximum switching frequency is limited to several hundred Hertz.
The present invention uses a piezoelectric method to actuate liquid metal switches. The actuator of the invention uses piezoelectric elements in a sheart mode rather than in a bending mode. A piezoelectric driver in accordance with the invention is a capacitive device which stores energy rather than dissipating energy. As a result, power consumption is much lower, although the required voltages to drive it may be higher. Piezoelectric pumps may be used to pull as well as push, so there is a double-acting effect not available with an actuator that is driven solely by the pushing effect of expanding gas. Reduced switching time results from use of piezoelectric switches in accordance with the invention.
A piezoelectrically actuated liquid metal switch in accordance with the invention is comprised of a plurality of layers. Liquid metal is contained within a channel in one layer and contacts switch pads on a circuit substrate. The amount and location of the liquid metal in the channel is such that only two pads are connected at a time. The metal is movable so that it contacts the center pad and either end pad by creating an increase in pressure between the center pad and the first end pad such that the liquid metal breaks and part of it moves to connect to the other end pad. A stable configuration results due to the latching effect of the liquid metal as it wets to the pads and is held in place by surface tension.
An inert and electrically nonconductive liquid fills the remaining space in the switch. The pressure increase described above is generated by the motion of a piezoelectric pump or pumps. The type of pump of the invention utilized the shearing action of piezoelectric elements in a pumping cavity to create positive and negative volume changes. These actions may cause pressure decreases, as well as increases, to assist in moving the liquid metal.